1. Field of the Invention
The present invention relates to conductive compositions which contain pigment grade carbon. More particularly, the invention relates to compositions suitable as primers and coatings in which pigment-grade carbons are utilized as the conductive component.
2. Description of Related
Coating compositions in which conductive carbon is the primary pigment material are well known. For example, U.S. Pat. No. 4,380,596 discloses solvent resistant primer compositions containing elemental carbon such as graphite or lampback. In U.S. Pat. No. 4,504,431, conductive pastes comprising small amounts of conductive carbon and large quantities of masking pigment in a 100% solids system are added to fiberglass and cured to provide electrically conductive articles. Compositions that provide conductive coatings are disclosed in U.S. Pat. Nos. 4,818,437 and 4,818,438; the conductive additive is a ground, calcined, coal-based coke which approaches graphite in performance.
While providing the desired conductivity, these formulations contain high ratios of resin to pigment. High pigment loadings are not used because of the cost of conductive carbons, the inability of such carbon-resin mixtures to form a continuous film free of microcracks and the formation of very viscous compositions when substantial amounts of conductive carbon are included in the formulation. Nevertheless, continuous coatings containing high pigment loadings are desirable in order to provide black coatings in many applications and to reduce the volatile organic compounds (VOC) per gallon of coating to meet the EPA regulations.
Various types of coating compositions using pigment grade carbons are also well known. Thus, U.S. Pat. No. 3,150,110 discloses high gloss aqueous paints containing small amounts of pigment grade carbon while U.S. Pat. No. 3,398,109 teaches solvent type black enamel coatings containing a carbon black which had been subjected to oxidative after-treatment. In U.S. Pat. No. 4,379,871 dispersable color concentrates based on a carbon black pigment powder are disclosed; the concentrates are used to prepare molded parts, castings, films, fibers, etc. However, these coating compositions have not been formulated for use in conductive coating applications.
Electrodepositable compositions containing anthracite and bituminous coals are disclosed in U.S. Pat. No. 3,658,737. These compositions function in the electrodeposition process because the coals do not provide conductive coatings.